This scenario usually indicates a problem with the hardware. To help isolate the problem, check the following items:

Overclocking: Disable overclocking to see whether the issue occurs when the system is run at the correct speed.

Check the memory: Verify the memory by using a memory checker. Verify that each memory chip is the same speed and that it is configured correctly in the system.

Power supply: Make sure that the power supply has enough wattage to appropriately handle the installed devices. If you added memory, installed a newer processor, installed additional drives, or added external devices, such devices can require more energy than the current power supply can provide consistently.

Overheating: Check whether the system is overheating by examining the internal temperature of the hardware.

Defaults: Reset the system back to the system defaults to see whether the issues occur when the system is running in its default configuration.

Power supply and Overheating were the first things that I thought of so you may want to start there.